Convection heating furnace for a tempered glass sheet

ABSTRACT

A convection furnace for a tempered glass sheet ( 1 ), into which furnace the glass sheet arrives along a hauling track, as on turning rolls ( 3 ), and the furnace has also heating resistances ( 5 ) against glass sheet ( 1 ) for heating the blast air, a blast apparatus and blast channelling ( 4 ), ( 2 ) for blasting said air against the glass sheet. The blast channelling comprises elongated channels ( 2 ) in the glass sheet ( 1 ) direction, inside of which there is at least a part of each blast air heating resistance ( 5 ) and each channel ( 2 ) has below the resistance line a broadening and in the broadening a bottom part ( 9 ), whereby bottom part ( 9 ) is furnished with blast holes ( 7, 8 ).

This application is a 371 of PCT/FI04/00182 filed 30 Mar. 2004.

The invention relates to a convection furnace for a tempered glasssheet, into which furnace the glass sheet arrives along a hauling track,as on turning rolls, further, the furnace has heating resistancesagainst the glass sheet for heating the blast air, a blast apparatus andblast channelling for blasting said air again the glass sheet.

Previously known from Patent specification EPO 0 910 553 B1 is atempering furnace for glass sheets as per the above preamble, whichfurnace has elongated air blasting channels. In these channels in thechannel direction heating resistances are fitted, which by means ofspecial radiation surfaces are adjusted mainly downwards against theglass. Blast air comes from the upper part of the channels towardsradiation canals partly surrounding the heating resistances and thepassing channels continue further downwards against special radiationplates furnished with holes. Due to the radiation channels the radiationplates receive strong heat radiation from the heating resistances and,in addition, the radiation plates are deliberately made quite thickplate panels so that the air would have time to get warm on flowingthrough the holes in them. The air flow does not rinse the heatingresistances and so their surface temperature is high and transmitsstrong heat radiation.

The disadvantage of the above described furnace is that the heating ofglass for the most part works as radiation, although even air isblasted, since the radiation plates in the solution are above the glassand considerably in much higher temperature than the temperature ofglass. Especially glasses coated in a certain way reflect radiation offand do not get warm by means of radiation heat. In connection with theheat resistances additional radiation canals must be made in thefurnace, and below resistances massive radiation plates with openingsare still slowly reacting to heat regulation Due to the use of heatresistances as radiation heaters, whereby they have a very high surfacetemperature, unavoidably results in shorter service life.

In order to eliminate the disadvantages of the above presented furnacesolution and to achieve a glass heating shape closer the completeconvection heating than before, a new convection heating furnace hasbeen developed, characterized in that the blast channelling compriseselongated channels in the glass direction or fitted at right anglesagainst it, inside of which, at least a part of each heating resistancethat heats the blast air is in free connection with the air flow andeach channel comprises below the resistance line a broadening and in thebroadening a bottom portion, whereby the bottom portion is furnishedwith blast holes.

The advantage of the invention is that heating takes place almostexclusively by means of convection, whereby the solution is suitable forall kinds of coated glass sheets. The heating resistances are directlyin strong air flow, whereby their surface temperature hardly rises muchover the blast air temperature. Accordingly, their service life getslonger and the impact of their radiation on the environment remains alsosmall. The air discharge nozzles are holes made in light platestructure. Substantially the plate structure is in the same temperatureas the strong air flow led through it, since the radiation of theheating resistances hardly heats the plate structure. Separatelyadjusted heating resistances can be fitted in the direction of the blastchannels or crosswise with respect to them There can also be heatingresistances fitted simultaneously into the furnace in both ways.

In the following the invention is disclosed with reference to theenclosed drawing, where

FIG. 1 shows a part of the heating furnace, where certain heatingcomponents on the glass sheet are shown diagonally from the side.

FIG. 2 shows the blast channel from the end.

FIG. 3 shows the assembly of a heating resistance fitted crosswise withrespect to the channel.

FIG. 1 shows a part of the of the glass sheet tempering furnace,comprising walls (not shown) and a hauling track, formed of rotatingrolls 3, on which glass sheet 1 can be moved during the process or afterit in a way wanted. To lead blast air onto the glass surface, there arein the furnace elongated channels 2, which are in this example fitted inglass sheet 1 direction. The blast air is taken to from one or severalblast apparatuses along distribution channellings 4. Placed insidechannels 2 are heating elements 5 in channel 2 direction. The channelsbroaden downward to the under side of heating elements 5 and end onbottoms 9 which have holes. At least the bottom portion is of thin plateand in it holes 7,8 are punched, most suitably so that with the punchingtool also collars downward (FIGS. 1-3) around the holes are made. Withthe thin plate a plate is meant, whose thickness is less than 3 mm.

Heating elements 5 in channel 2 are in relatively strong air flow andthe air flow runs closely past elements 5. That is why the elementseffectively release heat into the air and the temperature of theelements does not rise very much. The surface temperature in themremains substantially lower than in case of radiation heating, wherebythey have to send notable radiation power into the environment. Blastair runs past the resistance, gets warm therefore and continues in warmstate through the holes of bottom part 9 towards the glass. Due to thebroadened shape of bottom part 9 the coverage area of blast holes 7,8 inthe glass surface gets voluminous. Anyhow, between channels 2 sufficientspace remains for the air to get back up to the suction face of theblast apparatus.

FIG. 2 shows direct from the channel 2 end the fitting assembly of thechannels. The distance of bottom part 9 and glass 1 is adjusted to beabout 50-70 mm. bottom part 9 is far from heating element 5, distanceabout 70-120 mm. Therefore element 5 does hardly heat bottom part 9through radiation. Bottom part 9 reaches quite close to the blast airtemperature. The surface temperature of heating element 5 on its part isformed about 50-300° C. higher than the temperature to which the pastflowing air gets heated. The abovementioned difference of temperaturedepends greatly on the speed by which the air passes elements 5.

FIG. 3 shows an embodiment example, where in channel 2 crosswisedirection holes for heating elements 5 are formed and that in the spacebetween channels 2 there are protection bushings 6, adjusting thedistance of channels 2 and prevent elements from remarkable radiationout through the spaces between the channels. As to their shape andheight channels 2 also allow that there could be simultaneously alsoelements 5 in the glass direction, fitted both lengthwise and crosswise.The heat power of elements 5 at bushings 6 could be adjusted smaller,e.g. using fewer rotations of the resistance wire towards the unit oflength.

Below the glass sheet a heating arrangement as per FIG. 1 can be madeturned upside down and most suitably also turned so that channels 2 arein the direction of rolls 3 and in the spaces between them. Blast holes7 in the corners of bottom parts 9 are then towards rolls 3 and heatingthem. Also solutions of other kind for heating from the glass sheetunder side are possible, for instance ways of heating, where the portionof radiation is greater.

The invention is characterized in that the power of heating can beadjusted to different parts of the glass by means of heating elements 5,which can be separately adjusted. Most suitably elements 5 are side byside in the direction of elements, in other words in the traveldirection of the glass, whereby each of them has its own feed and thusindividual adjustment is also possible. If necessary, it is possible toget on glass 1 surface the accurate temperature distribution wantedmerely by adjusting of the heating elements.

In addition, by means of air distribution channels 4 it is also possibleto adjust the speeds of air and distribution of air quantities todifferent channellings 4 and this way also to influence the distributionof glass temperature. Also by means of general adjustment of the blastapparatuses it is possible to influence the nature of heating, in otherwords the portion of convection with respect to radiation. A greatquantity of air to be blasted and its speed move the way of heatingtowards almost perfect convection heating.

1. A convection furnace for a tempered glass sheet, comprising: ahauling rack along which the glass sheet is directed into the furnace,heating resistances which heat air to be blasted against the glasssheet, a blast apparatus including blast channelling for blasting theheated air against the glass sheet, wherein the blast channellingcomprises elongated air channels disposed in the glass sheet directionor at right angles to the glass sheet direction, wherein inside of eachsaid air channel there is at least a part of a said blast heatingresistance, wherein each said elongate air channel includes a narrowestportion in which the at least a part of said heating resistance islocated for effecting heat transfer by convection from each heatingresistance to the blast air, wherein each said air channel hasdownstream of the narrowest portion a broadening portion, saidbroadening portion including a bottom part furnished with blast holesthrough which the heated air is directed onto the glass sheet, andwherein said bottom part is made of a thin plate which, due to a strongconvection blast of the air and/or a surface quality chosen for saidthin plate, transmits substantially little thermal radiation to theadjacent glass sheet.
 2. A convection heating furnace according to claim1, wherein the air blast holes in the bottom part comprise collars madein the thin plate.
 3. A convection heating furnace according to claim 1,wherein a temperature difference between a surface temperature of theheating resistances and a temperature of the heated air is no more than300° C.
 4. A convection heating furnace according to claim 1, wherein atemperature difference between a surface temperature of a said heatingresistances and a temperature of the heated air is no more than 200° C.5. A convection heating furnace according to claim 1, wherein the bottompart temperature is substantially the same as the temperature of theblast air on the glass sheet.
 6. A convection heating furnace accordingto claim 1, wherein the heating resistances are located in a respectivesaid channel in the direction of the respective said channel.
 7. Aconvection heating furnace according to claim 1, wherein the heatingresistances are located crosswise with respect to said air channels andare led to travel through said air channels.
 8. A convection heatingfurnace according to claim 1, further comprising a separate adjustmentmechanism for power to said heating resistances for adjustment of thedistribution and rising speed of the class sheet temperature.
 9. Aconvection heating furnace according to claim 1, wherein said narrowestportion of said elongated air channel has a non-diverging cross sectionin an air flow direction.
 10. A convection heating furnace according toclaim 9, wherein said narrowest portion of said elongated air channelhas a constant cross section in the air flow direction.